Ion traps are known for electron discharge tubes, especially for use in cathode ray tubes as disclosed in U.S. Pat. Nos. 2,810,091; 2,836,752 and 2,921,212. The ion traps that are disclosed in these patents are used to prevent the impingement of negative ions that are carried together with an electron beam onto areas of the fluorescent screen in order to prevent darkening or discoloration of these areas which are defined as ion spots.
Positive ions are also created in electron discharge tubes and they are typically created by the impingement of electrons of the electron beams emanating from the heated cathode onto metal parts of the anode thereby creating positive metal ions or the electrons of the electron beam cause outgassing of gas molecules from the metal parts they engage which creates positive ions from these gas molecules or the electrons of the electron beam or secondary electrons therefrom create positive ions from free gas molecules that are present within the envelope of the electron discharge tube.
These positive ions can acquire kinetic energy in the vicinity of the cathode and impinge thereon thereby sputtering away cathode material and causing chemical poisoning which changes cathode structure and decreases the life of the cathode.